Dark matter obeys the same cosmic rules as ordinary matter, study finds

Most people rarely experience the effects of dark matter in ordinary life. However, we live in an environment affected by dark matter. You are sitting beneath stars that remain in place due to the force of dark matter, yet we are all moving through this universe that would cease to exist without motion.

However, no one has ever seen a particle of dark matter or felt one with their senses. Does this invisible mass follow the same rules that guide the ordinary matter that makes you and everything you know?

A recent study provides the most revealing answer to date. It indicates that dark matter travels through the cosmos in the same manner and following the same rules as normal matter. If you have ever pondered whether there is one set of rules in our universe or two, these findings bring you closer to an answer.

Scientists have mainly presumed dark matter interacts with matter solely via gravity. This straightforward explanation accurately depicts how galaxies spiral, how clusters appear, and how the universe expands over billions of years. In this view, dark matter drops into what is called a gravitational well in the same way stars and planets do, and respond to each other. Matter follows a rule, Euler’s equation, and normal matter adheres to the basic grasp of this equation established by gravity.

However, assumptions are not proof. As long as the rule feels stable, it provides comfort. However, uncertainties have led some researchers to ask whether there is something else at play. Does dark matter feel an additional pull from the universe that ordinary matter does not? Does gravity behave differently for different forms of matter?

These questions hit home for those hoping to get a fuller picture of the universe. New research, led by the University of Geneva with support from the University of Portsmouth, asks these questions directly.

Since dark matter cannot be seen or felt, scientists looked for dark matter's effect on the motion of galaxies and light. Scientists were able to follow the drift and merger of galaxies thanks to redshift, space distortions. Gravitational lensing showed the shapes and how deep the gravitational wells these galaxies are falling into.

These tools can provide a bridge between what is visible and dark matter. Redshift measurements show how fast cosmic structures are growing. Lensing maps show how deep the gravitational wells are. Together, they provide a glimpse of what may be pushing and pulling the galaxies moving through the universe.

The team collected velocity data from the results of 22 different surveys covering a broad range of distances. The lensing results from DES were also included in the team's findings, which were measured at four different redshifts. By applying a technique known as cubic spline interpolation to analyze these data, the researchers examined how galaxies changed velocity compared to how gravitational wells changed velocity. From this analysis, a quantity called Gamma was derived, which informs whether there is evidence of a fifth force (assuming it has a uniform strength across the distances being examined).

This number is significant. If Gamma equals zero, dark matter is falling only under the influence of gravity. If Gamma deviates from zero, an additional force is influencing dark matter's trajectory.

Throughout all four redshifts, Gamma equal zero was indistinguishable within the uncertainties. In other words, galaxies were behaving as if dark matter were only affected by gravity. The study found no evidence to show dark matter was impacted by any other force.

There were relatively tight constraints on the results. For the different redshifts measured, the uncertainties were between 0.17 and 0.29. Further, the tighter limits were obtained when the assumption of any added force was made to be constant for every observation - a fifth force could equal no more than seven percent of the strength of gravity in the positive sense and no more than 21 percent in the negative direction = anything larger would have appeared readily in the data.

Lead author Nastassia Grimm from the University of Portsmouth commented, "At this point, our conclusions do not eliminate the existence of an unknown force. However, if such a fifth force does exist, it cannot be stronger than seven percent of the strength of gravity."

There is a tremendous weight to that simple statement. In other words, it sounds like gravity continues to be the dominating influence on dark matter, as it does on you and everything in your world.

This study looked at a number of assumptions and kept standard cosmological physics intact. It relied extensively on earlier measurements, so the researchers will never get to probe dark matter as carefully as they could. But new surveys will offer a dramatic change in our ability to observe dark matter.

The Legacy Survey of Space and Time, run by the Vera C. Rubin Observatory and the Dark Energy Spectroscopic Instrument, will provide maps of galaxies and gravitational wells that are far more precise. Scientists expect to observe deviations from purely gravitational motion in each redshift bin at three to six percent precision with these tools. If they assume a constant force, they will then do as well as two percent.

Co-author Isaac Tutusaus from the University of Toulouse believes that this new data will allow for even more understanding of how dark matter behaves. This progress may allow dark matter interactions to be ruled out or reveal that they were otherwise hidden. For now, the study is significant as it shows dark matter appears to operate in a way that agrees with the behavior we expect due to gravity.

These results strengthen the underpinning of modern cosmology. By demonstrating that dark matter behaves like matter in our world, this study reinforces the models used to understand galaxy formation, cosmic evolution, and large-scale structure.

Together, tighter bounds on any new force help constrain future research into dark matter and increase the search for dark matter particles that the next survey phase looks toward. As this new survey provides better constraints, future science will learn if dark matter only follows gravity or could rely upon another leg of physics.

This knowledge would reshape how we think about the universe and influence new technologies that would depend on the more advanced gravitational theory and more meaningful high-precision measurements.

Research findings are available online in the journal Nature Communications.

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